oh, absolutely. I have a friend who's DD read the lot at aged 7/8 but she reckons she actually understood less than half of what's going on - though she still enjoyed them. JK Rowling 'advanced', if you like, her plots in accordance to the age of her original readers, so they become increasingly more complex - not like, say, Wimpy Kid where every book is at exactly the same level.
I have to say that as a huge HP fan I really dislike children being given them to read too young as they simply can't fully understand or appreciate the stories so that is colouring my response!